The present invention relates to a permanent magnet embedded rotating electrical machine which includes a stator having coils, a rotor rotatably located inside the stator and a plurality of flat permanent magnets embedded in the rotor as magnetic poles.
In this type of rotating electrical machine, a plurality of permanent magnets are embedded in the rotor of in the circumferential direction thereof in such a way that two adjacent magnetic poles of any two adjacent permanent magnets are different. Rapid change in magnetic flux density occurs between the two adjacent permanent magnets, that is, at a magnetic pole switching position. As a result, torque pulsation occurs, which causes vibration and noise in the rotating electric machine. Such torque pulsation will be referred to as torque ripple hereinafter. The torque ripple represents fluctuation range of output torque of the rotating electric machine.
Japanese Patent Application Publication No. 2005-261024 discloses a permanent magnet rotating electrical machine whose rotor core includes a pole piece located in an air gap surface of a permanent magnet and forming a magnetic path of the permanent magnet, an auxiliary magnetic pole that projects to the air gap surface at a position between any two adjacent permanent magnets of a plurality of permanent magnets for generating reluctance torque, a rotor yoke located in an anti-air gap surface of the permanent magnet and forming a magnetic path between the auxiliary magnetic pole and the permanent magnet, and a recess formed in the air gap surface of the pole piece and smoothly receding from the magnetic pole center toward the magnetic pole end. In order to suppress the torque ripple, the recesses formed on the opposite sides of the magnetic pole center are located in the range of electrical degree of 20° to 30° as measured from the magnetic pole center when the number of slots per pole and per phase of the stator is two. The recesses formed on the opposite sides of the magnetic pole center are located in the range of electrical degree of 15° to 45° as measured from the magnetic pole center when the number of slots per pole and per phase of the stator is one.
The range of electrical degree of the rotating electrical machine disclosed by Japanese Patent Application Publication No 2005-261024 is desirable in the case of the rotor where the permanent magnets are arranged in the shape of V (or where the permanent magnets are not arranged in rotational symmetry). However, such range of electrical degree cannot be applied to the rotor where flat permanent magnets are arranged in a block form (or where the permanent magnets are arranged in rotational symmetry).
The present invention is directed to a permanent magnet embedded rotating electrical machine having a plurality of flat permanent magnets having rotational symmetry and capable of suppressing torque ripple.